1. Field of the Invention
The present invention relates to an optical encoder.
2. Description of the Related Art
A triple grating measurement system (an optical encoder) has been conventionally used as a displacement measuring apparatus for obtaining displacement information of an object. The optical encoder can be downsized by adopting an optical element such as a laser diode (LD), a vertical cavity surface emitting laser (VCSEL), and an LED as its light source.
However, the characteristic of the laser diode or the vertical cavity surface emitting laser significantly is changed in accordance with a temperature change, and also has a problem that it is easy to be damaged by electrostatic discharge, it is very expensive, or the like. In addition, since luminous flux from the light source has high coherency, a lot of harmonic distortion components are contained in an output signal waveform. On the other hand, in the LED, spatial coherency is low and the distortion of the harmonic is small since a light emitting size is large, but it is difficult to obtain the amplitude of a fundamental wave signal. Recently, as a light source having a small light emitting window and being a high power, a current confinement-type LED becomes popular, and the spatial coherency is improved and detection at high resolution is possible.
In the optical encoder, various kinds of methods of eliminating a waveform distortion of a harmonic have been conventionally proposed. However, in a method of eliminating harmonic components having different orders at the same time by using one section, it is difficult to effectively eliminate the waveform distortion component and a remaining component is generated. Therefore, with respect to a harmonic component of each order, there is a method of applying different eliminating sections for each order.
In Japanese Patent No. 3045452 discloses a method of setting widths of a light transmissive portion and a non-transmissive portion of a second scale and a third scale to be a ratio capable of eliminating harmonics having N and N+1 orders, respectively, to eliminate distortion components of the orders that are to be eliminated by being divided by a first scale and the third scale. In the method disclosed in Japanese Patent No. 3045452, harmonics having different orders can be eliminated by the second scale and the third scale, respectively. Therefore, an optimum eliminating condition can be assigned for the harmonic component having each order, and a highly-accurate interpolation division signal can be obtained.
However, when the second scale is provided integrally with a light source to be downsized, a distortion component cannot be effectively eliminated and it is difficult to perform position detection with high accuracy because a light intensity distribution in the light emitting window of the light source is nonuniform. If the third scale is configured integrally with a light receiving element, it is necessary to arrange a lot of light receiving windows closely to each other in order to obtain a plurality of phase difference signals (for example, four-phase signals of A+, B+, A−, and B−). In this case, a distortion of the waveform caused by crosstalk of a plurality of signal sources or an offset error deteriorates the position detection accuracy of the encoder.